Inserting to table in VB.NET, LINQ to SQL - vb.net

Question

This is the beginnings of a timetabling algorithm. The problem is with inserting a member into a group, but I have included the whole subroutine here for context. The table "membergroup" has 2 headings, MemberID and GroupID. No error code is thrown, but the table does not receive the new record.
I have gone through it line by line, and the values for iMember_Choice.MemberID and groupID_to_insert are correct.
Dim numberofrooms As Byte = rm.Count
Dim possiblerooms(numberofrooms + 1), possibleroomcounter As Int32
For TimeTableNumber = 1 To Val(NumberOfTimetablesToCreate.Text)
Dim memchoic = (dc.ExecuteQuery(Of memberChoice)("SELECT * FROM MemberChoices ORDER BY NEWID()")).ToList 'Orders list randomly
'sort into array for each rank, so highest ranks are allocated first
Dim Member_choices_Table_ordered_by_rank = From q In memchoic Order By q.Rank
For Each Member_Choice In Member_choices_Table_ordered_by_rank
ProgressBar.Value = ProgressBar.Value + 1
Dim iMember_Choice As memberChoice = Member_Choice
'Dim memactpossibleinstance(maxmemchoicernk, n) As memactvpossibleinstance
For Each room In rm
Dim rmid As Int32 = room.RoomID ' finds rooms activities can be in
If Not (From rom In roomact Where rom.ActivityID = iMember_Choice.ActivityID And rom.RoomID = rmid).FirstOrDefault Is Nothing Then 'finds suitable rooms
possiblerooms(possibleroomcounter) = rmid
possibleroomcounter = possibleroomcounter + 1
End If
Next
'find possible times
Dim roomid_to_insert, current_maximum_rank As Integer
Dim period_to_insert As String = "MonAM"
Dim staffact_that_can_do_this_activity = _
(From q In staffact Where q.ActivityID = iMember_Choice.ActivityID)
For roomcount = 0 To possibleroomcounter - 1 'for each room in roomcount, find rank for room
Dim rmid As Int32 = possiblerooms(roomcount)
For Each time In periods
Dim itime As String = time.Period
Dim Rank As Int16 = member_Error_check_period_count(iMember_Choice.MemberID, itime)
If Not (From rav In rmav Where rav.RoomID = rmid And rav.Period = itime) Is Nothing Then 'room is free
Rank = Rank + 12
Else ' room has an activity
Dim GroupID_now = (From q In instnce Where q.Period = itime And q.RoomID = rmid Select q.GroupID).SingleOrDefault
If GroupID_now <> 0 Then
Dim groupActID_now = (From q In grp Where q.GroupID = GroupID_now Select q.ActivityID).SingleOrDefault
'Dim activity_now = (From q In actv Where q.ActivityID = groupActID_now).SingleOrDefault
If groupActID_now = iMember_Choice.ActivityID Then 'Good, this activity is already on in this room
Rank = Rank + 50
If (From q In memgrp Where q.GroupID = GroupID_now).Count > 4 Then
Rank = Rank - 50
End If
End If
End If
End If
If Rank > current_maximum_rank Then
current_maximum_rank = Rank
roomid_to_insert = rmid
period_to_insert = itime
End If
Rank = 0
Next
possibleroomcounter = 0
Next 'for each room possible
Dim groupID_to_insert As Integer
If (From q In instnce Where q.Period = period_to_insert And q.RoomID = roomid_to_insert).FirstOrDefault Is Nothing Then
groupID_to_insert = insert_ins_group(period_to_insert, roomid_to_insert, iMember_Choice.ActivityID)
Else
groupID_to_insert = (From q In instnce Where q.Period = period_to_insert And q.RoomID = roomid_to_insert Select q.GroupID).SingleOrDefault
End If
'PROBLEM PROBPBLY HERE/////////////////////////////////////
memgrp.InsertOnSubmit(New membergroup With {.MemberID = iMember_Choice.MemberID, .GroupID = groupID_to_insert}) 'PROBLEM PROBABLY HERE
dc.SubmitChanges()
current_maximum_rank = 0
Next 'for each memberchoice
dc.SubmitChanges()
Next 'timetabl no
dc.SubmitChanges()
memgrp is initiated as
Dim memgrp As Table(Of membergroup) = dc.GetTable(Of membergroup)()